As Beach Erosion Accelerates, Remedies Are Costly and Few

By CORY DEAN

Published: August 1, 1989

USING computerized measuring techniques, scientists are tracking with new precision the erosion that is eating away 90 percent of the nation's coasts.

Their findings are bad news, especially for the 295 barrier islands that lie like beads on a necklace along the coast from New England to Texas. The islands, long sandy strips of land immediately off the coast, are home to many of the nation's shore resorts and coastal towns.

The sea has been rising relative to the land for at least 100 years, geologists say, and most agree that erosion is going to accelerate as global warming melts the polar ice packs, sending sea levels even higher.

The barrier islands are adapting to the rising sea by slowly migrating to higher elevations inland; as their seaward dunes erode, new dunes rise just behind them. But in the process, structures newly exposed to the sea are often undermined. When heavy storms hit, dozens of houses can be destroyed, as happened on the outer banks of North Carolina in March.

Over the last 100 years, the Atlantic Coast has eroded on average 2 to 3 feet a year, said Stephen P. Leatherman, director of the Laboratory for Coastal Research at the University of Maryland. The average yearly erosion on the Gulf Coast is 4 to 5 feet, he said, and in some places, it is much worse. The south shore of Martha's Vineyard is losing about 10 feet a year, while Cape Hatteras, N.C., is losing 12 to 15 feet, and Louisiana is losing 30 to 50 feet a year.

''It's no secret any more what's happening on the coast,'' Dr. Leatherman said. ''Ten years ago we didn't have good data on erosion rates. Now we do, and we know about 90 percent of the coast is eroding.''

The threat has driven many communities to seek refuge behind seawalls and jetties, only to find that these costly engineering projects often make the erosion worse, either for them or for neighbors down the coast. Other resorts are spending vast sums to pump new sand onto their beaches, only to see the sand disappear again within a few years.

Many engineers maintain that seawalls, jetties and replenished beaches are necessary to protect the valued developments already on the barrier islands. Too many people live or vacation at Atlantic City, Miami Beach and other barrier island towns to let their roads and buildings simply fall into the sea, these experts say.

But many environmentalists and geologists contend that roads and buildings just do not belong on the unstable, migrating islands. At a minimum, some critics say, the Government should discourage further development on barrier islands by refusing to subsidize such projects as roads and waste treatment plants, or property insurance for owners. Seawalls Saving Buildings, Losing the Beach Although geologists say that nothing can stop the sea's inexorable march inland, few barrier beach communities are prepared to let themselves fall into the ocean block by block. Often, they try to halt erosion with bulkheads or rocky seawalls that protect the structures behind them.

But many scientists now say that such structures accelerate the erosion of sand on the beaches in front of them.

''Seawalls destroy beaches,'' said Orrin H. Pilkey Jr., a Duke University geologist. In a recent study of scores of sites on the Atlantic Coast, Dr. Pilkey and his colleagues found that beach width on stabilized beaches was dramatically lower than on beaches with no sea walls. Many of the stabilized ''beaches'' had no beach at all.

Although the mechanics of wave action on beaches are imperfectly understood, geologists can suggest ways in which a seawall might hasten erosion. On a gently sloping beach, a wave's force is reduced as the surge of water, or swash, rushes onshore. Much of the water's energy is gone by the time it returns down the seaward slope as backwash, so it carries off relatively little sand.

A seawall cuts this process off. Water hits it sharply and is reflected back strongly, carrying sand with it.

''The wave hits the seawall and the energy doesn't dissipate,'' Dr. Leatherman said. ''A seawall is a last Draconian step to save property. You just kiss off the beach.''

Sea Bright, N.J., for example, once had 300 feet of beach in front of its massive seawall, said Gilbert Nersesian, a coastal engineer for the New York District of the Army Corps of Engineers. ''But the beach that existed in front of that seawall is completely gone.''

Maine and North Carolina have passed laws banning new hard stabi, lization along the shoreline. But property owners pay a heavy price for such laws. In March, a series of storms off of North Carolina eroded the beaches at Nags Head and Kill Devil Hills, on the Outer Banks, pitching several houses into the sea and damaging others so severely they were condemned.

''We let a lot of houses fall in,'' Dr. Pilkey said. ''I'm proud of that.'' But houses threatened here and there are not the truest test of whether a state's politicians can withstand pressure for stabilization, he said.

Pointing to a five-story condominium on the dune line on Bogue Bank, a heavily developed barrier island in North Carolina, he asked: ''Are we going to let a building like this fall into the sea?'' Stopping Sand Flow Feeding One Beach By Starving Another For centuries, people have built jetties of stone jutting out from the shore to keep sand from silting up channels. And for years they have tried to stem beach erosion by building smaller groins out from the shore to trap the sand carried along shore by currents.

Now there is wide agreement that groins can only hold sand on one beach by starving another downstream.